Saul sent to smite Amalek in fulfilment of God's command

Nevertheless Saul is put to a final proof. Jehovah, by the mouth
of Samuel, sends him to smite Amalek, and utterly destroy them and
all that belonged to them. They were the cruel and determined
enemies of God's people (Deut. 25: 17-19). They had been chief among
the nations, their name and their pride were everywhere known
(Num. 24: 7, 20); but it was a nation doomed of God.

Saul's disobedience and rejection

God now entrusts Saul with the fulfilment of Deuteronomy 25:
19. In this case all Israel accompanied him without fear. These were
not the enemies from within who were daily wearing away their
strength and courage. The victory is complete. The only question now
is that of faithfulness to God, and of preferring His glory to
self-interest. But Saul fears the people. The Spirit of God says
"Saul and the people"; Saul says "the people"; and that it was for
God they spared. But our excuses, even when true, only condemn
us. Saul, not having faith, not looking to God, fears the people
more than God. What a slave is the unbeliever! If not the slave of
the enemy, he is that of the people whom he appears to govern. Saul,
unfaithful to God in the midst of the people, and surrounded by
blessings from Jehovah, is at length deprived of the kingdom.

No humiliation, no brokenness of heart -- he confesses his sin,
hoping to avoid its punishment; but, unable to escape it, he
entreats Samuel to honour him in spite of it. Samuel does so and
then forsakes him. Everything changes now, and David appears on the
scene. It is well to remark that the connected history of Saul's
reign closes with the end of chapter 14.

Chapter 15 is given as a separate history on account of the
importance of its contents -- the definite rejection of Saul, a
rejection which introduces David.